1. Field of the Invention
The present invention relates generally to multifocal diffractive lenses, and more particularly relates to a multifocal diffractive lens which exhibits maximum possible intensities in the main diffractive powers and with optimum contrast performance.
2. Description of the Prior Art
Diffractive bi- and multifocal lenses are known in the art. Reference is made to the Cohen patent family and to the patents by Freeman, Futhey, Taboury and Isaacson which are discussed below.
The Cohen patent family on diffractive lenses encompasses in essence the following patents: U.S. Pat. No. 4,210,391; 4,338,005; 4,340,283; 5,054,905; 5,056,908; 5,117,306; 5,120,120; 5,121,979; 5,121,980; 5,144,483; and European Patent Application EP 0367878.
U.S. Pat. No. 4,210,391 discloses a multifocal zone plate which has first focal power means in odd zones for directing incident parallel light to a first focal point and second focal power means within even zones for directing incident parallel light to a second focal point different from said first focal point.
U.S. Pat. No. 4,338,005 discloses a phase plate construction in which the first and second focal points are specified and where the zone radii are defined as a function of a single wavelength which is termed "the wavelength under consideration". No refractive powers of the zones are discussed.
U.S. Pat. No. 4,340,283 discloses a multifocal zone plate construction in which the zone radii are defined as a function of a single wavelength and of the difference between the first and second focal powers. No refractive powers within the zones are discussed.
U.S. Pat. No. 5,054,905 discloses an ophthalmic lens comprising a carrier lens having an anterior surface and a posterior surface and a zone plate disposed about an optical axis of the carrier lens on at least one of the anterior and posterior surfaces. The zone plate includes a plurality of annular zones, wherein each of the zones has at least one echelette and each of the echelettes has a surface. The surfaces of each of the echelettes is defined as having non-uniform profiles.
U.S. Pat. No. 5,056,908 discloses an ophthalmic contact lens with a phase plate and a pure refractive portion within its optic zone.
U.S. Pat. No. 5,117,306 discloses a diffractive bifocal lens or lens system comprising a first profiled surface or interface and a second profiled surface or interface which provides for positive chromatic aberration to be associated with a first focal point and negative chromatic aberration to be associated with a second focal point.
U.S. Pat. No. 5,120,120 discloses a multifocal optical device which comprises a multifocal phase zone plate including at least two annular zones and absorbing means disposed on a portion of selected ones of the annular zones for absorbing a portion of light. The device is used for focusing light traveling parallel to an optical axis of the device.
U.S. Pat. No. 5,121,979 discloses a bifocal lens utilizing phase zone plate optics with blazed facets. Each of the facets are blazed with the same angle and a facet depth of one-half wavelength of the design wavelength.
U.S. Pat. No. 5,121,980 discloses an optical device that employs a phase zone plate comprising a plurality of blazed facets. No discussion of the blazing of the facets or the refractive powers of the facets is disclosed.
U.S. Pat. No. 5,144,483 discloses a bifocal lens utilizing phase zone plate optics and a facet depth of one-half wavelength of the design wavelength. The multiple focal point profiled phase plate having a plurality of annular concentric zones in which a repetitive step is incorporated into the profile between blazed facets. Again, a discussion of facet blazing or refractive power of the facets is not present in the disclosure.
European Patent Application EP 0367878 discloses multifocal lenses using phase shifting steps. The multiple focal point profiled phase plate includes a plurality of annular concentric zones in which a repetitive step is incorporated in the profile.
The zones, echelettes or facets of the aforementioned Cohen diffractive lens design are not zone lenses with a specified geometric power. The disclosed Cohen lenses exhibit irregularities or steps on at least one of the lens surfaces. In use, these irregularities fill with tear fluid causing interference. Mention is made that considerable effort is spent in geometric step or blaze design of diffractive lenses in order to minimize as much as possible the discomfort of lenses with alterations on the lens surface. Cohen's lens designs with smooth surfaces are occasionally discussed; however, such smooth surfaces are exclusively achieved by ion implantation within the zones. In most of the Cohen lens designs, the light incident on the lens is parallel to the optic axis of the lens. One of the Cohen lens designs discloses a refractive power.
Freeman's patent family on diffractive lenses consists in essence of the following patents: U.S. Pat. Nos. 4,637,697; 4,642,112; 4,655,565; and 4,641,934.
U.S. Pat. No. 4,537,697 discloses a bifocal contact lens which has diffractive power added to the basic refractive power. A first portion of the light is focused by refraction, and a second portion of said light is focused by asymmetric zone plate surfaces.
U.S. Pat. No. 4,642,112 discloses an artificial eye lens having basic refractive power providing one focus, and having diffractive power which deviates some light from the basic refractive power focus to another focus.
U.S. Pat. No. 4,655,565 discloses a contact, spectacle or implant lens which has positive diffractive power which introduces negative longitudinal chromatic aberration. No refractive powers of diffractive lens zones are discussed.
U.S. Pat. No. 4,641,934 discloses an ophthalmic lens which incorporates a transmission hologram having negative diffractive power which introduces positive longitudinal chromatic aberration.
Freeman's disclosed diffractive lens designs either exhibit a refractive power and an added diffractive power, or a diffractive power which introduces longitudinal chromatic aberration. No geometric powers are associated with the zone plate surfaces.
U.S. Pat. No. 4,830,481 to Futhey, discloses a multifocal ophthalmic lens which has a plurality of concentric diffractive zones with adjacent zones separated by steps having predetermined heights. No geometric powers are associated with the concentric diffractive zones, and steps separate adjacent zones.
U.S. Pat. No. 4,936,666 to Futhey discloses a lens having diffractive power, which is produced by a plurality of diffractive zones, the diffractive zones being terminated by optical steps. A first group of the diffractive zones has optical heights equal to j.lambda. and a second group of the diffractive zones has optical heights equal to k.lambda., where .lambda. is the design wavelength of light of the lens and j and k are unequal nonzero integers. No geometric powers are associated with the concentric diffractive zones, and steps separate adjacent zones.
U.S. Pat. No. 5,129,718 to Futhey discloses a diffractive lens in which the central zone is smaller than the other zones. A meniscus lens and a biconvex lens are disclosed (FIGS. 8 and 9) which exhibit smooth outer surfaces. No discussion is dedicated to the shape of these smooth surfaces, and no discussion is present about the refractive powers of the zones.
U.S. Pat. No. 5,229,797 to Futhey discloses a lens with diffractive power and two primary foci which are associated with the first and the second diffractive order. It is stated (column 6, lines 12-14) that "the radius of curvature of the base curve is a parameter in determining the optical powers of the multifocal diffractive lens". As will be shown, infra, the base curve is not the determining factor for the optical powers of a multifocal lens according to the present invention.
U.S. Pat. No. 5,104,212 to Taboury discloses a diffractive contact lens in relief including a smoothing layer. The smoothing layer is on the back surface of a contact lens with a spherical back surface. The front surface "may be constituted, for example, by a portion of a spherical surface centered on a point lying on the lens axis" (column 2, lines 40-42). No discussion of the refractive powers of the lens zones is present in this disclosure.
U.S. Pat. No. 5,152,788 to Isaacson, discloses a multifocal ophthalmic lens including a first and second lens member welded together and forming an inner cavity. A diffractive zone plate is positioned on the inner surface of one lens member. The lens members have convex, planar or concave outer surfaces, depending upon refractive optical contribution desired by the lens member. The inner surface may be planar or convex (column 4, lines 49-51). No discussion is dedicated to the shape of the inner and outer surfaces within individual zones, and, as a consequence, to thc individual refractive powers of the zones.
European Patent Application EP 0468410A1 to Shiono ct al., discloses a diffractive optical lens comprising a grating zone having a plurality of elliptical grooves. The lens is used to correct astigmatism. No annular lens zones of refractive power are disclosed, and, the surface of the grating zone exhibits grooves, i.e. it is not smooth.
German Patent No. DE 4134518A1 to Stork, discloses a saw-toothed diffractive lens (FIG. 5) and in particular an IOL in which the saw-toothed profile is superimposed on the common plane interface of two plano-convex lenses of different indices of refraction (FIG. 10). No annular zone lenses of refractive power are disclosed, no exact dimensions for the saw-toothed profile are given, which dimensions are not trivial since the light rays at the planar interface traverse the lens at varying angles to the lens axis. No other interface than planar between saw-toothed profiles of lenses of different indices are disclosed, and no dimensioning for the zone blazes is given.
Usually, a diffractive lens consists of any number of annular lens zones of equal areas, so-called Fresnel zones. At the common border of adjacent Fresnel zones usually +.lambda./2-steps or -.lambda./2-steps are introduced between all zones, .lambda. being the so-called design wavelength, in order to provide for constructive interference of light waves in the 0th and 1st diffractive order, or the 0th and -1-st diffractive order, respectively. Also, diffractive lenses are known (Futhey) in which the steps between adjacent zones are any odd integer half of the design wavelength.
Besides designs in which the optical steps between zones are always positive or negative, designs with +.lambda./2-steps and -.lambda./2-steps between subsequent adjacent zones are known. In such lenses, constructive interference of light waves takes place predominantly in the -1st and +1st diffractive order. Additionally, Cohen has disclosed designs in which the steps between adjacent zones assume still other values.
Although the theory of diffractive lenses is well established and the theoretical performance of such lenses appears to be satisfactory, diffractive bi- and multifocal lenses have not found wide application e.g. in ophthalmology. This fact may be due to deficiencies of practical embodiments of such lenses, as will be explained, infra.